The isotropic phases of four fractions of a mesogenic comb-shaped polymer were studied using the electric birefringence method. The fractions have either nematic – isotropic melt, or smectic-A – isotropic melt phase transition, depending on the degree of polymerization. All samples showed a structural transition in isotropic phase. Mathematical modeling of the electro-optical properties was performed based on the modified Landau-De Gennes theory using three- order parameters. The reason for the transition in the isotropic phase is the mutual rearrangement of the polar side groups of the polymer as a result of dipole-dipole interaction.